Reduced temperature curing of acetylenic polymers

ABSTRACT

The invention provides a method for promoting the curing reactions of an acetylenic oligomer or polymer, characterized in that the oligomer or polymer is cured in the presence of sulfur or an organic sulfur derivative which is capable of lowering the temperature of cure of the oligomer or polymer. The invention also provides a process for producing a polyimide oligomer or polymer containing an aliphatic or aromatic disulfide moiety which is covalently bound to, and forms an integral part of the oligomer or polymer and which is capable of lowering the temperature of cure of the oligomer or polymer, characterised in that a suitable amount of a diamino-disulfide or dianhydride-disulfide, or any suitable derivative or precursor thereof, is introduced into the mixture of aromatic diamines, tetracarboxylic dianhydrides, and the phenylethynyl-substituted amine or anhydride normally used for the reparation of the oligo-imide.

This invention relates to a method for lowering the curing temperatureof acetylenic substituted polymers. The method of the invention isparticularly, but not exclusively, concerned with the curing ofacetylenic polyimides.

Aromatic polyimides are widely used as resins for moulding of plasticsarticles, as adhesives, and as matrices for composite materials intendedfor service at elevated temperatures. Such polyimides are generallyproduced by the condensation of a mixture of one or more diarnines witha stoichiometric amount of one or more tetracarboxylic dianhydrides in asuitable solvent such as dimethyl formamide or N-methylpyrrolidone toform a polyamic acid which on heating can undergo cyclodehydration toform the polyimide (for example, as shown in FIG. 1 of the accompanyingdrawings). Often the intermediate polyamic acid solution is used forcoating the articles which are then heated to form the polyimidein-situ.

Whilst polyimides of high molecular weight are required for thedevelopment of adequate mechanical strength, it is often preferred, forease of production and other reasons, to employ a lower molecularweight, oligomeric imide or amic acid containing substituents which canundergo chain extension and crosslinking reactions during thermalprocessing to form the cured thermoset resin. Examples of such systemsalready widely used in industry include the bismaleimides and thenadimide-based PMR resins which undergo cure at temperatures near 250°C. Many other systems have been described in the literature, but thesehave apparently failed to obtain commercial acceptance.

One disadvantage of most of these thermoset polyimides is their failureto withstand oxidative degradation on long-term exposure at temperaturesabove 200° C. This is because the crosslinking moieties have generallyinferior thermal stability, compared to the oligoimide units, andtherefore can act as weak links in the polymeric structure.

One class of thermoset polyimides which does appear to provide adequatethermal stability are those containing phenylethynyl-substitutedaromatic species as the reactive moieties, e.g.:

Such systems have been described in patents assigned to National Starch(e.g., U.S. Pat. No. 5,138,028) and the United States NationalAeronautics and Space Administration (e.g., U.S. Pat. No. 5,567,800).However, the processing of these phenylethynyl resins requires curing atmuch higher temperatures (350-400° C.) than those typically required forthe cure of the bismaleimide or nadimide resins.

The chemistry involved in the curing of the phenylethynyl(phenylacetylenic) resins has not been conclusively established, but isbelieved to involve the initial condensation of two or more ethynyl(acetylenic) groups to form a mixture of linear poly-enes or ene-yneswhich can then undergo a variety of inter- and intra-molecular additionand substitution reactions to form the crosslinked, fully cured product.The initiation of these reactions probably involves adventitious freeradical intermediates, although the ethynes (acetylenes) as a class arenot particularly susceptible to radical-induced polymerization.

It is therefore conceivable that processing temperature for thephenylethynyl resins could be lowered, for example, by the addition offree radical catalysts such as peroxide initiators, or by utilising theknown activation of alkynes by transition metal complexes. However,these approaches have some potential difficulties.

To obtain void-free products, it is desirable that the resin should befully cyclized before onset of the cure reactions, and should be free ofsolvents, amic acid residues, and other species which could evolvevolatile by-products during processing. However, the fully cyclized,solvent-free oligo-imides which could serve as precursors for curedresins having the preferred glass transition (Tg) temperatures in excessof 250° C. themselves usually have softening or Tg temperatures above200° C. and the chain extension and crosslinking reactions cannotproceed at practical rates below the softening point or Tg of the resin.Most peroxidic initiators undergo rapid and irreversible decompositionwell below these temperatures, and so would be inefficient as cureaccelerators for practical phenylethynyl resins. The use of transitionmetal catalysts, on the other hand, would leave undesirable residueswhich could also promote thermooxidative degradation of the cured resinduring service at elevated temperatures.

We have now discovered that the addition of suitable organic disulfidesor polysulfides, or elemental sulfur, to phenylethynyl-substitutedoligo-imides reduces the onset of cure temperature by 50° C. or more,thus enabling the thermal curing of the resins to occur at temperaturesat 300° C. or below. We have also found that additional improvements canfollow from the structural inclusion of disulfide moieties inoligo-imide chains. These additives or disulfide moieties undergoreversible dissociation at 200° C. or above, preferably 200° C. to 300°C., to form thiyl radicals which can react with the phenylethynyl groupsto initiate the cure of the resin.

The use of sulfur, disulfides, and polysulfides as additives in thevulcanization of olefinic elastomers is well known. However, thereappears to be no previous reports or claims for their use as curingagents in acetylenic polyimides, or more specifically, the commerciallypromising phenylethynyl polyimides.

According to the present invention there is provided a method forpromoting the curing reactions of an acetylenic oligomer or polymer,which comprises curing the oligomer or polymer in the presence of sulfuror an organic sulfur derivative which is capable of thermally generatingthiyl radicals during the curing reaction thereby lowering thetemperature of cure of the oligomer or polymer.

The organic sulfur derivative can be selected from disulfides andpolysulfides of the formula

R—S_(n)—R′

wherein (n≧2) and the substituents R and R′ may be substituted orunsubstituted alkyl, cycloalkyl, aryl, arylalkyl, or heterocyclicmoieties, and may be the same or different; or derivatives thereof, suchas mono- or di-acyl or aroyl disulfides of the formula:

imidyl (imidoyl) or thiocarbamyl disulfides of the formula:

wherein R and R′ are as defined above; or other sulfur-containingspecies, including sulfenyl derivatives and elemental sulfur, which cangenerate thiyl radicals when heated to the processing temperature of theresins, typically in the range of 150-300° C.

In another aspect the invention provides an acetylenic oligomer orpolymer having at least one ethynyl group, characterised in that itcomprises an organic sulfur moiety which is covalently bound to, andforms an integral part of the oligomer or polymer and which is capableof thermally generating thiyl radicals during cure of the oligomer orpolymer thereby promoting the cure of the oligomer or polymer.

In a further aspect the invention provides a composition which comprisesan acetylenic oligomer or polymer having at least one ethynyl group andsulfur or an organic sulfur derivative having an organic sulfur moiety,characterised in that the sulfur or organic sulfur derivative is capableof thermally generating thiyl radicals during cure of the oligomer orpolymer thereby lowering the temperature of cure of the oligomer orpolymer.

In yet another aspect there is provided a process for producing anacetylenic polyimide oligomer or polymer containing one or more ethynylgroup per molecule and containing an aliphatic or aromatic disulfidemoiety which is covalently bound to, and forms an integral part of theoligomer or polymer and which is capable of lowering the temperature ofcure of the oligomer or polymer, characterised in that a suitable amountof a bis(amino-substituted)hydrocarbyl disulfide orbis(anhydride-substituted)hydrocarbyl disulfide, or any suitablederivative or precursor thereof, is introduced into the mixture ofaromatic diamines, tetracarboxylic dianhydrides, and thephenylethynyl-substituted amine or anhydride normally used for thepreparation of the oligomer or polymer.

In this specification “substituted” group means that a group issubstituted with one or more non-deleterious groups selected from:alkyl, alkenyl, aryl, halo, haloalkyl, haloalkenyl, haloaryl, hydroxy,alkoxy, alkenyloxy, aryloxy, haloalkoxy, haloalkenyloxy, haloaryloxy,amino, alkylamino, alkenylamino, alkynylamino, arylamino, acyl, aroyl,alkenylacyl, arylacyl, acylamino, heterocyclyl, heterocyclyoxy,heterocyclylamino, haloheterocyclyl, alkoxycarbonyl, alkylthio,alkylsulphonyl, arylthio, arylsulphonyl, aminosulphonyl, dialkylamino,dialkylsuphonyl.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the reaction for the formation of a polyamic acid and,subsequently, a polyimide.

FIG. 2 shows a curing reaction according to the invention, where thiylradicals are generated to cure acetylenic oligomers and polymers.

FIG. 3 shows a reaction using arylthiyl-generating oligomers to preventthe loss of active species through volatilization.

In the above, the “aryl” moiety may be phenyl or another mono- orpoly-carbocyclic aromatic ring system optionally substituted with one ormore alkyl, amino, haloalkyl, halo, or cyano groups.

In the above, the “heterocyclyl” moiety is a 5-8 membered ringcontaining one to three hetero atoms such as oxygen, nitrogen or sulphurand may be substituted and/or carry fused rings and which may or may notbe aromatic or pseudo-aromatic. Examples of such groups includespyrrolidinyl, morpholinyl, thiomorpholinyl, or fully of partiallyhydrogenated thienyl, furanyl, pyrrolyl, thiazolyl, oxazolyl, oxazinyl,thiazinyl, pyridinyl and azepinyl.

In the whole context to define alkyl or cycloalkyl but if “alkyl” meansstraight chain or branched C₁-C₃₀ alkyl, and “cycloalkyl” means C₃-C₁₂cycloalkyl, for example cyclopropyl, cyclobutyl, cyclopentyl,cyclohexyl, cycloheptyl and the like.

Any substituent should not interfere with the cure of the oligomer orpolymer.

The preferred organic sulfur derivatives are long-chain alkyl disulfidessuch as di-n-dodecyl disulfide, arylalkyl disulfides such as dibenzyldisulfide, or aryl disulfides such as diphenyl disulfide, ornon-volatile, fusible oligoners containing dithioalklyl or dithioarylgroups. When one of these disulfides, for example, di-n-dodecyl ordiphenyl disulfide, is added to a phenylethynyl terminated oligo-imide,such as those used as precursors for high temperature resins, the onsettemperature of the curing reactions is reduced. In the case of the modelcompound, N-phenyl 4-phenylethynylphthalimide, differential scanningcalorimetry (DSC) shows that the onset temperature of the curingreactions is reduced from 300° C. to below 250° C., and the peakexotherm temperature from 400° to 300° C. Fully cured resins can beobtained by heating at 300° C. to 320° C., whereas full cure of the sameresin without additives requires cure temperatures above 360° C.

The course of these reactions and the properties of the products dependson the chemical structure of the disulfides and their derived thiylradicals. In the case of alkyl and benzyl disulfides, studies of modelcompounds have indicated that the intermediate thiyl radicals initiallyreact with one or more phenylethynyl groups to form a mixture of labilethiosubstituted vinylic and linear oligo-enes which are believed toundergo a series of further reactions to form the cured product. In thecase of simple aromatic disulfides such as diphenyl disulfide (see FIG.2, structure I), the intermediate thio-vinyl radical (i.e., the monomeradduct, III) formed from the initial attack on the phenylethynyl groupby the thiyl radical (II) can also undergo a facile back-biting reactionwith the latter to form thermally stable benzothiophenes (IV),as well asreacting with a second phenylethynyl group to form a dimer adduct (V).The latter can also undergo a backbiting reaction to form a mixture oftetra-aryl-substituted thiophenes (VI), as well as propagating to formhigher poly-enes. Polyenes appear to be also formed indirectly throughthe aromatisation reactions which accompany the benzothiophene andthiophene formation. As a consequence of these backbiting reactions, theinitial products from the arylthiyl radical induced reactions tend tohave lower molecular weights and more linear structures than thoseformed from alkyl- or benzylic-thiyl radicals.

The thiyl radical generating moieties can also be incorporated as partof the ethynyl-containing oligo-imide chain. This prevents the possibleloss of active species through volatilization during storage orprocessing, and also can result in more-effective chain extension andmolecular weight build-up, particularly in the case of oligomerscontaining arylthiyl generating moieties (see FIG. 3). Althoughdisulfides are preferred, the thiyl generating moieties may be analoguesof any of the other additive types listed above. The products from thesereactions differ significantly in both structure and properties fromthose formed in the cure of their disulfide-free analogues. The formerare more lightly crosslinked and more thermoplastic in character thanthe latter; they have been found to be significantly tougher inconsequence, but to have slightly lower Tg than the latter.

These disulfide containing oligomers can be readily prepared by theincorporation of a suitable amount of a diarnino-disulfide ordianhydride-disulfide into the mixture of aromatic diamines,tetracarboxylic dianhydrides, and the phenylethynyl-substituted amine oranhydride normally used for the preparation of the oligo-imide.Formation of the intermediate disulfide-containing acetylenic oligo-amicacids and their thermal cyclodehydration in solution can be achievedwithout any significant premature reaction between the disulfide andphenylethynyl moieties. Alternatively, the disulfides can be formed bythrough reactions of substituents on monomeric or oligomeric components,for example, the aerial or chemical oxidation of mercapto-groups or thecondensation reactions of mercaptans with sulfenyl derivatives.

Suitable diamino-disulfides can include aliphatic species, preferablythose containing primary disulfides, such as2,2′-dithiobis-1-ethanaminie (cystamine), 3,3′-dithiobis-1-propanamine,1,1-dithiobis-2-propanamine, and higher aminoalkyl disulfides, as wellas cycloaliphatic, aromatic, benzylic, and heterocyclicdiamino-disulfides. For good thermal stability of the cured resins,aromatic diarnino-disulfides are preferred, particularly bis(amino-aryl)disulfides such as 3,3′- and 4,4′-dithiobis(1-benzenamines) (aminophenyldisulfides, dithioanilines), or their derivatives bearing alkyl, aryl,alkoxy, aryloxy, halo, or other substituents:

dithiobisnaphthylamines and substituted dithiobisnaphthamines, orextended species such as dithiobis(arylenoxy-arylamines),dithiobis(arylenecarbonyl-arylamines), dithiobis(arylenecarbonyl-iminoarylamines), or other species having the followinggeneral structure

H₂N,—Ar′—Q—Ar″—S.S—Ar′—NH₂

H₂N—Ar′—Q—Ar″—S.S—Ar″—Q—Ar′—NH₂

where Ar′ and Ar″ are the aromatic or substituted aromatic moietiesbearing the amino and disulfide groups respectively; Q is a linkingmoiety which may be a single bond, a simple group such as hydrocarbyl,ether, thioether, carbonyl, ester, tert-amino, amide, imide or sulfone,or a more complex unit containing a series of linked aliphatic,alicyclic, aromatic or heterocyclic species, i.e. it may be oligomeric.

Preferred dianhydride-disulfides are aromatic compounds such5,5′dithiobis(1,3-isobenzofurandione) (dithiodiphthalic anhydride) anddithiodinaphthalic anhydrides, their substituted derivatives, or otherspecies having the general structure

where Ar′ is the moiety bearing the dicarboxylic anhydride group, andAr″ and Q are as described above.

Whilst the dithiodiamines or dithiodianhydrides can be used directly inthe oligo-imide formulation, it is also possible to use derivatives suchas amine salts or tetracarboxylic acids, esters, or hemi-esters, orother derivatives which can undergo displacement reactions duringprocessing to form oligo-amidic or oligo-imidic products.

The molar ratio of the disulfide, as additive or moiety, relative tophenylethynyl content of the oligo-imide may lie between 2 and 12 moleof phenylethynyl groups per mole of disulfide additive. When theproportion of disulfide exceeds 1 mole per 2.5 mole of phenylethynylmoieties, free mercaptans derived from the parent disulfide may remainin the cured resin. If the proportion of disulfide is less thanapproximately 1 mole per 4 mole of phenylethynyl moieties, then the cureat temperatures of 300-320° C. is incomplete, leaving residualphenylethynyl groups; these can still undergo further, conventionalcuring reactions when the partially cured resin is heated to 360° C. orabove. Whilst full cure may not be obtained when disulfide-starvedformulations are heated at low temperatures, the degree of reaction maybe adequate for practical purposes, and may also provide a useful andreproducible means for obtaining a partially cured or “B-staged” resin.

B-staging of resins can be advantageously achieved by the use ofphenylethynyl-substituted oligomers containing aromatic disulfides asbackbone moieties. On heating at moderate temperatures, below 300° C.for example, cleavage of the disulfide moieties and their subsequentreaction with the phenylethynyl groups results in the formation of ahigher molecular weight thermoplastic polymer believed to containbenzothiophene linkages (see FIG. 3), together with a proportion ofpolyenemoieties which can undergo further reactions when heated athigher temperatures, above 300° C., to form a cured, lightly crosslinkedproduct.

The oligomers should preferably contain at least two acetylenic moietiesper oligomer chain. These may form the terminal units as depicted in thefollowing structural outline (where [S_(x)] represents the thiylgenerating species):

or they may be incorporated as substituent species or in chain-branchingmoieties along the chain to provide pendant ethynyls:

or as ethynyl links within the chain:

or any combination of these.

Each ethynyl group should be linked to at least one aromatic ring, butpreferably to two rings to form a diarylacetylenic moiety. Examples ofthe latter include phenylethynyl-phthalimide, phenylethynyl-anilide, orphenylethynyl-phenoxyanilide terminal units:

derived by the incorporation into the oligo-imide formulation of thecorresponding substituted phthalic anhydride or anilines having thefollowing structures, where R may represent a hydrogen, or an alkyl,aryl, or other substituent, and AR′ is the aromatic moiety bearing theethynyl group; Q, Ar″, and R are linking and substituent groups definedabove:

Non-terminal acetylenic moieties can be derived by the addition to theoligo-imide formulation, for example, of ethynyl- orphenylethynyl-substituted dianhydrides or diamines:

whilst in-chain diarylethynyl links can similarly result from theinclusion of ethynediyl-bisanhydrides or ethynediyl-bisamines:

The composition of the remainder of the oligo-imide is not critical, andis determined by the required properties of the cured resin. The(number) average molecular weight of the oligo-imide may range from 1000to 15,000, but the preferred range is 2,500 to 7,500. For materialsintended for use at high temperatures, the formulation should be basedon aromatic diamines and dianhydrides in order to achieve adequatemechanical properties and thermal stability. The oligomeric resin shouldhave a melting point or glass transition temperature below the curingtemperature to enable the curing reactions to proceed at acceptablerates. To obtain a suitable processing window and to improve thesolubility of the oligomer, if required, the diarnines or dianhydridesmay need to contain a proportion of extended (multi-nuclear) specieshaving one or more diaryl-ether, -thioether, -carbonyl, or -sulfonyllinkages, and possibly also methyl or phenyl substituent groups. Thecompositions may also include a proportion or tri- or higher amines oranhydrides to form branched chain oligomers. These amines or anhydridesmay be simple species or may themselves consist of oligomericcondensates bearing two or more free amino or anhydride substituents.

Methods for the preparation of oligomeric imides are well known.Typically, a mixture of one or more diamines is reacted with one or moredianhydrides in a suitable solvent to form a oligo-amic acid. Theaverage molecular weight of this oligo-amic acid is determined by thestoichiometry, and material free from terminal amine or anhydridespecies and having a defined molecular weight can be obtained by theincorporation of a proportion of monoamine or monoanhydride which thenform the terminal moieties. In the present invention, to form anethynyl-terminated oligomer for example, the monoamine or monoanhydridewould be one of the acetylenic derivatives listed above, whilst for adisulfide-containing oligomer, the diamines or dianhydrides wouldinclude a suitable proportion of a disulfide-substituted species toprovide the required statistical ratio of ethynyl to disulfide groups.

The oligo-amic acid can be converted to a fully cyclized oligo-imide bydehydration, preferably by refluxing the solution and removing theby-product water as an azeotrope. The solvent mixture may consist of anamidic solvent such as N-methylpyrrolidone containing a minor amount ofan azeotroping solvent such as toluene or xylene. Depending on theintended application, the oligo-imide solution may be used directly as avarnish, or it may be used to impregnate a fibrous tow, tape, or cloth,and then dried to form a prepreg for the production of laminatedarticles. Alternatively, the oligomer can isolated by precipitation onthe addition of its solution to a non-solvent such as water or methanol,followed by washing and drying to remove residual solvents.

The oligo-imide may be used alone, when it includes disulfide-containingmoieties, or it may be blended with the requisite quantity of adisulfide additive prior to curing the resin. Although the principalapplication of these oligomers may be as a matrix resin in theproduction of fibre-reinforced laminates, the fully-cyclized resin couldalso be suitable, with or without the addition of fillers orreinforcements, for the production of compression-moulded articles.

The invention also includes applications of the method of the invention,such as the production of a fibre-reinforced prepreg, in which thedisulfide-containing oligo-amic acid solution is used to impregnatereinforcing fibres, with conversion to the oligo-imide by dehydrationoccurring in-situ during subsequent processing. It also includesapplications in which a dry disulfide-containing oligo-imide is used toimpregnate a fibrous tow, tape, or cloth to form a prepreg. It furtherincludes applications in which the prepreg is heated, before or afterlaying up and consolidation of the laminated article, to convert(B-stage) the oligomeric matrix into a higher molecular weight,essentially thermoplastic polymer.

Curing of the oligomers can be achieved by conventional methods, byheating the neat resin or prepreg laminate, with consolidation underpressure to eliminate voids. The cure temperature required depends onthe composition of the oligomer, but should be above 200° C. fordissociation of the disulfide to occur. For phenylethynyl resinscontaining an optimum amount of disulfide, fully cured materials can beobtained by heating to within 20° C. of the glass transition temperatureof the product. The disulfide-acetylene curing chemistry can also beadvantageously applied to acetylenic polymers other than polyimides. Thecure of phenylethynyl-substituted polyetheretherketone has been found tomirror that of the polyimides, and this invention could be applied tothe processing other aliphatic or aromatic condensation oligomerscontaining acetylenic substituents into high molecular weight orcrosslinked polymers. These other oligomers include those containinghydrocarbyl, ether, sulfide, sulfone, carbonyl, ester, amide, andphosphine oxide linking moieties, or a combination of these, with orwithout imide moieties.

The invention is further described and illustrated by the non-limitingexamples which follow. The following abbreviations are used for thenames of chemical ingredients and solvents.

PEPA 4-phenylethynylphthalic anhydride BPADA2,2-bis-[4-(3,4-dicarboxyphenoxy)phenyl]-propane dianhydride. sBPDA3,3′,4,4′-tetracarboxybiphenyl dianhydride ODPA 4,4′-oxydiphthalicanhydride TMMD 2,5,6-trimethyl-1,3-phenylene diamine 3,4′-ODA3,4′-oxydianiline 4,4′-ODA 4,4′-oxydianiline mPDA 1,3-phenylene diamineTPE-R 1,3-bis-(4-aminophenoxy)benzene APDS 4-aminophenyl disulfide TMTDtetramethylthiuram disulfide DMAc dimethylacetamide NMPN-methylpyrrolidone THF tetrahydrofuran

All temperatures are reported in degrees Celsius.

Differential scanning calorimetry (DSC) was used in the study of thecuring reactions, the samples being heated at a rate of 10° C./min to450° C. in a nitrogen atmosphere. DSC was also used for thedetermination of the glass transition temperature (Tg) of the curedresins. Gel permeation chromatographic analysis (GPC) in THF was usedfor determination of the number-average (Mn), weight-average (Mw), andelution-peak (Mp) molecular weights, using polystyrene standards forcalibration.

EXAMPLE 1

A pure phenylethynyl-terminated oligo-imide was prepared by the reactionof N-(3-aminophenyl)4-phenylethynylphthalimide (2.8 g) with BPADA (1.51g) in DMAc (10 ml), the intermediate bis-amic acid being cyclized bytreatment with acetic anhydride (2.5 ml) and triethylamine (0.7 ml) at60°. The product was recovered after precipitation into water. The dryacetylenic imide had a molecular weight of 1160 and melted at 165°. DSCanalysis indicated a curing exothern (208 J/g) commencing near 320° witha peak at 389°.

DSC analysis, heating in air, of a mixture containing 20 parts by weightof the acetylenic oligomer and 1 part of sulfur, equivalent to anethynyl/“disulfide” ratio of 2.2:1, showed an endotherm with a peak near140°, corresponding to the melting point of the sulfur, followed by aexotherm which commenced near 200° with peaks at 234° and 335° C. Themagnitude of the second exotherm (over 1000 J/g) indicated that it mayhave been due to oxidation or other reactions of excess sulfur.

EXAMPLE 2

A mixture was prepared containing 5 parts of the acetylenic oligomer ofExample 1 and 1 part of TMTD. DSC analysis of the mixture indicated amelting point near 180° followed by an complex exotherm having peaks at244° and 362°.

EXAMPLE 3

A mixture was prepared containing 10 parts of the acetylenic oligomer ofExample 1 and 1 part of diphenyl disulfide, corresponding to anethynyl/disulfide ratio of 3.8:1. DSC analysis of the mixture indicateda curing exotherm which commenced near 210°, with a peak near 279°, anda second weaker peak near 390°; the latter is believed to result fromcuring of residual ethynyl groups remaining after the variousdisulfidelacetylene reactions which occurred in the main below 300°.There was also some evidence of the loss of diphenyl disulfide byvolatilization from the unsealed system during the heating cycle.

EXAMPLE 4

A non-volatile disulfide-containing oligomer was prepared as follows.

4-Aminophenyl disulfide (APDS) was prepared by the reaction of4-nitrochlorobenzene (110 g) with a refluxing solution of sodiumhydrosulfide (160 g) in water (1 L) for 16 hr, followed by distillationto remove by-product 4-chloroaniline, approximately 150 ml of aqueouscondensate being collected. The mixture was then cooled to 50° andtreated with 17% aqueous hydrogen peroxide (200 ml). The crudeamninophenyl disulfide was recovered by filtration and purified, firstby refluxing with a 10% aqueous solution of sodium sulfite to decomposethe aminophenyl trisulfide co-product, and then by recrystallisationfrom 400 ml of 1:1 aqueous ethanol. The yield of pure product was 74 g.

Phthalic anhydride (2.96 g) was added, with stirring, to a solution ofTMMD (3.0 g) and APDS (4.96 g) in NMP (100 ml) at 20° under N₂. BPADA(16.14 g) was then added and the mixture stirred for a further 1 hr.Toluene (20 ml) was then added and the mixture heated under reflux,using a Dean-Stark trap to collect the water separated by azeotropicdistillation. When water evolution had ceased, after 3.5 hr reflux, themixture was cooled and the oligomer isolated by precipitation intomethanol. The oligomer had a disulfide molar weight of 1300 and startedto melt near 210°.

A phenylethynyl-terminated oligomer designed for high temperatureservice was prepared by the reaction of a mixture of PEPA (7.25 g), TMMD(7.41 g), mPDA (5.33 g), BPADA (21.0 g), and sBPDA (12.89 g) in 150 mlNMP. Toluene (30 ml) was added to the intermediate oligo-amic acidsolution and the mixture heated under reflux for 5 hr, using aDean-Stark trap to remove the by-product water. The resultant solutionwas cooled and the oligomer recovered by precipitation into water andthen washed with hot methanol to remove the toluene and residual NMP.The oligomer has an ethynyl molar weight of 1800; DSC analysis indicatedthat melting started near 250°, and the material had a curing exothermcommencing near 300°, with a peak at 397°. The oligomer could be fullycured by heating at 360° for 4 hr, and the cured resin had a Tg of 302°.

Finely ground mixtures of the disulfide and acetylenic oligomers wereprepared by treating a slurry of the two in a vibratory ball mill; theintimately mixed material was recovered by filtration and dried invacuum at 180°. The mixtures contained 2, 4, 6, and 8 parts (by weight)of acetylenic oligomer per part of disulfide oligomer; thesecorresponded respectively to 1.4, 2.9, 4.3, and 5.7 ethynyl groups perdisulfide.

DSC analysis of the 2:1 mixture showed that curing exothern commencednear 240°, with a peak at 309°. The 4:1 mixture showed a curing exothermhaving two connected peaks, one near 265° corresponding to the meltingpoint of the acetylenic oligomer, the other occurring at 3200. The 6:1oligomer showed exotherm peaks near 260° and 332°, with a third minorpeak near 397°. The 8:1 oligomer also showed three exotherm peaks, thepeak near 260° being weak, whilst the major peak occurred at 389°; thelatter is believed to correspond to cure reactions of the excessphenylethynyl groups.

The mixed oligomers were also cured by heating under pressure in ahydraulic press. The powders were first consolidated at 250°, thenheated to 320° over 1 hr and held at that temperature for 4 hr beforeslowly cooling to room temperature. The 2:1 and 4:1 cured resins werebrittle, and the former had a sulfurous smell. DSC analysis indicatedthat the Tg's of the polymers were: 2:1, 245°; 4:1, 282°; 6:1, 282°;8:1, 283°. The cured 6:1 and 8:1 resins also showed an exothermic peaknear 360°, indicative of an incomplete cure at 320°; rescan of the 8:1sample showed that heating during the DSC analysis had fully cured thespecimen, raising the Tg to 294°.

EXAMPLE 5

A phenylethynyl-terminated, disulfide-containing oligomer was preparedas follows. mPDA (5.72 g) and PEPA were dissolved in anhydrous NMP (100ml) under N2 at 20°. APDS (1.74 g) was then added with stirring,followed by BPADA (26.00 g). After 10 hr, toluene (15 ml) was added andthe mixture heated to reflux. The by-product water was collected in aDean-Stark trap, small aliquots of toluene being added to maintain therefluxing mixture at a pot temperature of 160-165°. Water evolutionceased after 5 hr, after which the mixture was cooled and theoligo-imide recovered after precipitation into methanol. The oligomerhad a ethyne/disulfide molar ratio of 3:1 and a molecular weight (Mn) of3630. DSC analysis showed a curing exothern starting at 250°, with apeak at 300°.

The oligomer was heated at 180° under vacuum for 3 hr to remove residualsolvent and then packed into a mould for curing. The mould was placed ina hydraulic press preheated to 200°. The powder was consolidated underpressure as the temperature was increased to 250° over 20 min. Thetemperature was then increased to 3000 over 50 min and then maintainedat this temperature for 4 hr before slowly cooling to room temperature.The cured resin was mechanically tough, free from any sulfurous odour,and insoluble in solvents. DSC analysis indicated the material had a Tgof 232° and showed no measurable exotherm on heating to 450°, i.e., itwas fully cured.

EXAMPLE 6

A low-melting oligomer containing a 2.85:1 molar ratio of ethynyl groupsto disulfide, and soluble in THF, was prepared by the general method ofExample 5, using as reagents: 5 3,4′-ODA (10.60 g), PEPA (449 6 g), APDS(1.74 g), and BPADA (26.00 g). The oligomer had Mn, 3800; Mw, 8046; andMp, 7834. DSC analysis indicated that the onset of the cure exothermoccurred at 260°, with a peak at 314°.

A disulfide-free oligomer was also prepared using the reagents andmethod above, but substituting the APDS with 4,4′-ODA(1.40g). Onset ofthe DSC exotherm of the oligomer occurred at 320°, with a peak at 395°The oligomer had Mn, 3796; Mw, 8391; and Mp, 8396, indicating thatnegligible pre-reaction between the disulfide and phenylethynyl moietiesoccurred during the synthesis of its disulfide-containing analogue.

The disulfide-containing oligomer was first freed from residual solventby heating at 180° under high vacuum to remove residual solvent, andthen aliquots were heated in sealed tubes for times and at temperatureslisted in Table 1. The molecular weights of the initial (dried) oligomerand its products are listed in the Table, together with the Tg, exothermpeak temperature (Tp) and enthalpy (H) values.

TABLE 1 Heat Mn* Mw* Mp* Tg (° C.) Tp (° C.) H (J/g) Initial 3.9 8.5 8.4171 314 42 204°/8 hr 4.8 10.5 10.5 160 313 37 250°/2 hr 8.5 19.8 18.9179 335 4 250°/4 hr 8.8 25.5 22.0 183 338 1 300°/1 hr 13.8 42.9 40.2 192— — 300°/2 hr 15.1 58.8 43.0 197 — — 300°/4 hr 17.2 68.5 49.4 — — —320°/1 hr 15.7 57.1 44.5 — — — *(÷1000)

The tabulated data indicates that the oligomer is relatively stable attemperatures below 200°, but is readily transformed into a highermolecular weight, thermoplastic (soluble) polymer when heated at 250°.Heating for longer periods at 320°, or for higher temperatures resultsin the formation of a crosslinked, insoluble product. The enthalpyvalues (H) are an indicator of the residual reactivity of the productafter the specified heat treatment. The late chain extension andcrosslinking reactions occur without any apparent change in enthalpy; inthis respect, the cure of the disulfide containing oligomer resemblesthat of its sulfur-free analogue.

EXAMPLE 7

A disulfide-containing oligomer analogous to that of Example 6, butcontaining a phenylethynylidisulfide molar ratio of 4:1 was prepared bythe method of Example 5 using the following reagents: 3,4′-ODA (11.00g), PEPA (4.96 g), APDS (1.24 g), and BPADA (26.00 g). Aliquots of theoligomer were treated as described in Example 6; the properties of theproducts are described in Table 2.

TABLE 2 Heat Mn* Mw* Mp* Tg (° C.) Tp (° C.) H (J/g) Initial 4.6 9.4 8.8170 308 (420) 58 204°/8 hr 4.2 10.0 10.4 171 318 (424) 51 250°/2 hr 8.318.5 17.4 183 (336) 415 17 250°/4 hr 7.6 20.0 19.1 187 418 13 300°/1 hr10.2 30.2 22.9 194 405 14 320°/1 hr 11.8 49.9 24.9 198 415 13 *(÷1000)

In this case of this disulfide-starved oligomer, the reactions in occurin to distinct stages, the low temperature, disulfide-mediated which canoccur readily at 250°, and appear as an DSC exotherm with a peak near300°, and the reactions of the residual ethynyl groups which only occurat much higher temperatures, and appear as a second, overlappingexotherm with a peak above 400°. Samples heated for longer periods at320° or, particularly, 360° or above became crosslinked and insoluble.The Tg of the fully cured resin was 209°.

EXAMPLE 8

Samples of the disulfide-free oligomer of Example 6 were blendedrespectively with di-n-dodecyl (aliphatic), dibenzyl (benzylic), anddiphenyl (aromatic) disulfides to provide mixtures containing aphenylethynyl/disuefide molar ratio of 3:1.

Portions of the additive-free oligomer and that containing the aromaticdisulfide were heated in sealed tubes at 250° for 15 hr. The propertiesof the products listed in Table 3.

TABLE 3 Oligomer Mn* Mw* Mp* Tg (° C.) Tp (° C.) H (J/g) Unheated 4.29.1 9.4 165 395 63 No additive 5.2 12.8 11.4 170 395 47 Blend 4.7 10.710.5 161 286 4 *(÷1000)

The tabulated data show that the disulfide-free oligomer undergoes somedegree of chain extension, probably initiated by adventitiousimpurities, when heated at 250°. In the presence of the aromaticdisulfide, the oligomer reacts to a much greater extent, but the bulk ofthese reactions at 250° are believed to result in the conversion of thephenylethynyl end-groups of the oligomer into benzothiophene moietieswhich, in the case of a simple disulfide such as diphenyl disulfide,results in little change in molecular weight.

The oligomer blended with the aliphatic disulfide, on DSC analysis,showed an complex exotherm which commenced near 260° and had a peak at295° with a shoulder near 370° which tailed to 400°, the greater part ofthe enthalpy change occurring after the 295°. The increase in molecularweights on heating aliquots at different temperatures are shown in Table4.

TABLE 4 Heat Mn* Mw* Mp* 250°/5 hr 9.0 18.3 17.9 300°/2 hr 10.4 23.121.6 320°/2 hr 13.7 45.7 25.5 *(÷1000)

The initial reactions of the aliphatic disulfide with the oligomer at250° are believed to result in a larger proportion of polyenes than thereactions with an aromatic disulfide, reflected by the greater increasein molecular weights on heating. The further increases in molecularweight are believed to result from the chain extension and branchingreactions of these intermediate polyenes. This example indicates thatthe addition of simple aliphatic disulfides can catalyse the cure ofdisulfide-free oligomers to form potentially useful products.

The oligomer blended with the benzylic disulfide, on DSC analysis,showed an exotherm which commenced at 230°, with a major peak at 268°and secondary peak near 360°. The increase in molecular weights onheating aliquots are shown in Table 5.

TABLE 5 Heat Mn* Mw* Mp* 250°/5 hr 7.0 16.6 16.3 300°/2 hr 9.3 25.5 21.8320°/2 hr 9.5 34.9 20.9 *(÷1000)

The onset of the cure exotherm of the oligomer blended with dibenzyldisulfide occurs at a lower temperature than that containing thealiphatic or aromatic sulfides; reflecting the more ready dissociationof the benzylic disulfides. Like the aliphatic disulfides, the monomericbenzylic disulfides also result in a significant molecular weightincrease of the oligomer on heating the blend at low temperatures.

EXAMPLE 9

An phenylethynyl-terminated oligomer containing aliphatic disulfidemoieties was prepared by the general method of Example 5 using thefollowing reagents: 4,4′-ODA (5.21 g), PEPA (2.48 g), BPADA (13.00 g),bis-(2-aminoethyl) disulfide dihydrochloride (0.90 g), and triethylamine(0.81 g) in NMP (75 ml) and toluene (25 ml). The oligomer had Mn, 1790;and Mw, 2681. The oligomer on heating at 250° for 5 hr yielded amechanically tough, crosslinked product which was swollen by, butlargely insoluble in THF. However material heated at 300° or 320° for 5hr was soluble in THF, its average molecular weights decreased withincrease in cure temperature, probably reflecting the lower thermalstability of the aliphatic linkages. DSC analysis showed that the cureexotherm commenced at 265°, with a peak at 298°.

EXAMPLE 10

A phenylethynyl-terminated, disulfide-containing, fully-cyclizedoligo-imide having a ethynyl/disulfide molar ratio of 3:1 and having atarget Mn of 5000 was prepared by the general method of Example 5 usingas reagents 3,4′-ODA (11.94 g), PEPA (3.72 g), TPE-R (3.36 g), SBPDA(20.22 g), and APDS (1.24 g) in 150 ml of NMP. A sulfur-free analogue ofthis formulation was also prepared by replacing the APDS with anequivalent molar amount of 3,4′-ODA (1.00 g).

DSC analysis of the sulfur-free oligomer showed that it had a Tg at 215°(the resin melted near 270°) with an exotherm peak at 407°. Superimposedon the exotherm was a sharp endotherm at 363° corresponding to thesemicrystalline-melting point of the curing polymer. The oligomer, aftercuring at 360° for 3 hr yielded a tough, high temperature-, andsolvent-resistant polymer having a Tg of 280° and a broadsemicrystalline melting endotherm near 380°.

DSC analysis of the disulfide-containing oligomer showed that it had aTg near 227° (the resin melted near 265°), with a complex cure exotherm.This showed an initial minor peak near 250°, with the main peakcommencing near 280° and reaching a maximum near 355°. Samples whenheated rapidly up to above the resin melting point could be cured toprovide resins having Tg's of 260-270° but with a semicrystallinecomponent analogous to that observed in the sulfur-free resin. DSCstudies showed that the development of this crystallinity inhibited thecure of both types of resin unless the cure temperature was above thesemi-crystalline melting point of the matrix. These tests also indicatedthe rapid advancement of the sulfur-containing oligomer on slow heatingfrom 250° to higher temperatures prevented the development of a lowviscosity melt desirable for composites processing.

EXAMPLE 11

The composition of the disulfide-containing oligomer of Example 10 wasmodified to limit the potential development of semi-crystallinity duringcure and also to improve the resin flow characteristics by substituting25 mole-% of the sBPDA dianhydride component with ODPA. The modifiedresin was prepared by the general method of Example 5, and contained asreagents 3,4-ODA (23.94 g), PEPA (7.44 g), TPE-R (6.68 g), sBPDA (30.33g), ODPA (10.66 g), and APDS (2.48 g) in NMP (300 ml), with the additionof toluene (60 ml) as an azeotroping agent. DSC analysis indicated thatthe oligomer had Tg 197°, the exotherm peak was at 342°. The resinflowed readily at 230-250°, and could be cured by heating at 320° for 4hr to form a tough material having a Tg of 260°; DSC analysis indicatedthat the cured resin was amorphous. The cured resin was tough, and theweight loss on long term ageing in air at 250° was very low, andcomparable to that of the sulfur-free product of Example 10.

A similar formulation to that above, but containing only 15% of thedianhydride component as ODPA yielded an oligomer having a Tg of 220°which, on curing at 320°, provided an amorphous product having a Tg of269°. However, the higher softening temperature of the oligomer resultedin the more rapid advancement of the resin and greater melt viscositiesthan those obtained using the oligomer containing 25% of ODPA.

EXAMPLE 12

A low-melting model phenylethynyl-terminated poly(etheretherketone)(PEEK) was prepared by the condensation of4-phenylethynyl-4′-fluorobenzophenone (5.0 g) with2,2-bis(4-hydroxyphenyl)-hexafluoropropane (2.8 g) by heating underreflux with potassium carbonate (2.5 g) in a mixture of DMAc (45 ml) andtoluene (35 ml). DSC analysis of the product melted at 160° and had acure exotherm which commenced at 320°, with a peak at 403°. A sample ofthe product was blended with diphenyl disulfide to provide a mixturehaving a ethynyl/disulfide molar ratio of 3:1. DSC analysis of thismixture indicated a cure exotherm which commenced at 210°, with a peakat 304°.

This example demonstrates that the promotion of the cure of thephenylethynylaryl moieties by disulfide is independent of the nature ofthe substituents on the aryl group.

An attempt was made to prepare a higher molecular weightdisulfide-containing PEEK oligomer using the method above by theaddition of APDS and difluorobenzophenone to the formulation. However,the mixture was found to be sufficiently alkaline to hydrolyse thedisulfide bonds during the oligomer synthesis. In devising analternative synthetic route for the oligomer, strongly acidic mediawould also need to be avoided as these can convert the disulfide tosulfenium ions which can undergo facile addition to the acetylenicmoieties.

What is claimed:
 1. A method for promoting the curing reactions of anacetylenic oligomer or polymer, characterised in that the oligomer orpolymer has at least one ethynyl group and is cured in the presence ofsulfur or an organic sulfur derivative which is capable of thermallygenerating thiyl radicals during the curing reaction thereby loweringthe temperature of cure of the oligomer or polymer.
 2. A method asclaimed in claim 1, characterised in that the organic sulfur derivativeis selected from disulfides and polysulfides of the formula R—S_(n)—R′wherein (n≧2) and the substituents R and R′ may be substituted orunsubstituted alkyl, cycloalkyl, aryl, arylalkyl, or heterocyclicmoieties, and may be the same or different; and derivatives thereof. 3.A method as claimed in claim 1, characterised in that the organicsulphur derivative is a mono- or di-acyl or aroyl disulfide of theformula:

wherein R and R′ are as defined in claim
 1. 4. A method as claimed inclaim 1, characterised in that the organic sulphur derivative is a mono-or di-acyl or aroyl disulfide of the formula:

wherein R and R′ are as defined in claim
 1. 5. A method as claimed inclaim 1, characterised in that the organic sulphur derivative is animidyl (imidoyl) or thiocarbamyl disulfide of the formula:

wherein R and R′ are as defined in claim
 1. 6. A method as claimed inclaim 1 or claim 2, characterised in that organic sulfur derivative islong-chain alkyl disulfide, an arylalkyl disulfide or an aryl disulfideor a non-volatile, fusible oligomer containing dithioalkyl or dithioarylgroups.
 7. A method as claimed in claim 1, characterised in that thesulfur or organic sulfur derivative is mixed with the acetylenicoligomer or polymer before curing.
 8. A method as claimed in claim 1,characterised in that the organic sulfur derivative is covalently boundto, and forms an integral part of the acetylenic oligomer or polymer. 9.A method as claimed in claim 1, characterised in that one mole of theorganic sulfur derivative is present for each 2 to 12 moles of ethynylgroups present in the oligomer or polymer.
 10. A method as claimed inclaim 1, characterised in that the acetylenic oligomer or polymercontains two or more ethynyl groups per oligomer or polymer chain.
 11. Amethod as claimed in claim 1, characterised in that the ethynyl groupspresent in the oligomer or polymer are disubstituted.
 12. A method asclaimed in claim 11, characterised in that the substituents are arylgroups.
 13. A method as claimed in claim 9, characterised in that theethynyl groups comprise the terminal groups of the oligomer or polymer.14. A method as claimed in claim 9, characterised in that the ethynylgroups are pendant substituents on the oligomer or polymer.
 15. A methodas claimed in claim 9, characterised in that the ethynyl groups formpart of the backbone of the oligomer.
 16. A method as claimed in claim9, characterised in that the oligomer is a condensation polymer.
 17. Amethod as claimed in claim 15, characterised in that the oligomer is apoly-imide having a molecular weight in the range 1000 -15,000.
 18. Anacetylenic oligomer or polymer having at least one ethynyl group,characterised in that it comprises an organic sulfur moiety which iscovalently bound to, and forms an integral part of the oligomer orpolymer and which is capable of thermally generating thiyl radicalsduring cure of the oligomer or polymer thereby promoting the cure of theoligomer or polymer.
 19. A composition which comprises an acetylenicoligomer or polymer having at least one ethynyl group and sulfur or anorganic sulfur derivative having an organic sulfur moiety, characterisedin that the sulfur or organic sulfur derivative is capable of thermallygenerating thiyl radicals during cure of the oligomer or polymer therebylowering the temperature of cure of the oligomer or polymer.
 20. Anoligomer, polymer or composition as claimed in claim 18 or 19,characterised in that it includes an organic sulfur derivative having anorganic sulfur moiety derived from an aliphatic or aromatic disulfide.21. An oligomer, polymer or composition as claimed in claim 18 or 19,characterised in that one mole of the organic sulfur moiety is presentfor each 2 to 12 moles of ethynyl groups present in the oligomer orpolymer.
 22. An oligomer, polymer or composition as claimed in claim 18or 19, characterised in that the acetylenic oligomer or polymer containstwo or more ethynyl groups per oligomer or polymer.
 23. An oligomer,polymer or composition as claimed in claim 18 or 19, characterised inthat the oligomer is a poly-imide having a molecular weight in the range1000-15,000.
 24. A process for producing an acetylenic poly-imideoligomer or polymer containing one or more ethynyl group per moleculeand containing an aliphatic or aromatic disulfide moiety which iscovalently bound to, and forms an integral part of the oligomer orpolymer and which is capable of lowering the temperature of cure of theoligomer or polymer, characterised in that a suitable amount of abis(amino-substituted)hydrocarbyl disulfide orbis(anhydride-substituted)hydrocarbyl disulfide, or any suitablederivative or precursor thereof, is introduced into the mixture ofaromatic diamines, tetracarboxylic dianhydrides, and thephenylethynyl-substituted amine or anhydride normally used for thepreparation of the oligo-imide or polymer.
 25. A process as claimed inclaim 24, characterised in that a dithiodiamine or dithiotetracarboxylicanhydride, or any suitable derivative or precursor thereof is used. 26.A composition or composite material characterised in that it comprisesan oligomer, polymer or composition as claimed in claim 18 or
 19. 27. Areinforced article characterised in that the composition of claim 26 isused to form the matrix for preparation of the article.
 28. A reinforcedarticle as claimed in claim 27, characterised in that the reinforcementis a fibre tow, tape or cloth.
 29. A fibre-reinforced prepreg,characterised in that it comprises an oligomer, polymer or compositionas claimed in claim 18 or
 19. 30. A process for forming a thermoplastichigh molecular weight polymer which on heating at temperatures above300° C. can undergo further reactions to form a crosslinked, thermosetarticle, characterised in that an acetylenic oligomer, polymer orcomposition as claimed in claim 18 or 19, or a resin-reinforcementmixture containing such an oligomer, is heated at temperatures of 200°C. to 300° C.
 31. A method according to claim 1 or 2 wherein the organicsulfur derivative is an acetylenic oligomer or polymer having at leastone ethynyl group, characterised in that it comprises an organic sulfurmoiety which is covalently bound to, and forms an integral part of theoligomer or polymer and which is capable of thermally generating thiylradicals during cure of the oligomer or polymer thereby promoting thecure of the oligomer or polymer.